Apps for a mobile device like a smartphone can use the geographic location/position of the mobile device as implicit input for a recommender system. Thus, for example, a homepage (or launcher) app might list the entities such as restaurants that are in the vicinity of the smartphone device.
The problem with such implicit input is that it might not reflect the present interests of the mobile device's user. So, for example, the homepage app might list restaurants when the user is actually more interested in motels at a particular point in time.
On the other hand, obtaining explicit input as to a mobile-device user's interests tends to be inconvenient for the user in terms of such things as time spent doing data entry.
Consequently, recommender systems based on implicit input from a mobile device remain an active area of research and experimentation.